X-ray CT devices are devices for reconstructing a tomographic image (henceforth referred to as CT image) of a subject by using X-ray transmission image data of the subject obtained by imaging with revolving a pair of oppositely disposed X-ray tube and X-ray detector on both sides of the subject, and they are widely used in the field of diagnostic imaging, and so forth. Since velocity of the aforementioned revolution in such X-ray CT devices is limited, they suffer from a problem that false images called motion artifacts are generated in CT images of moving subjects such as the heart, and degrade diagnostic ability. For solving this problem, the electrocardiography-gated scan was devised, and put into practical use. The electrocardiography-gated scan is a method of performing X-ray CT imaging with monitoring electrocardiographic waveform by using an electrocardiograph attached to a subject. In this method, a short time gate width is provided around a predetermined cardiac phase of the heart repeating a periodic motion, and image reconstruction is performed by using only imaging data obtained within this time gate width. Therefore, motion artifacts can be reduced.
The electrocardiography-gated scan is classified into two types, the prospective scan and the retrospective scan, on the basis of the difference in data acquisition scheme (refer to Non-patent document 1). The prospective scan is a method in which X-ray is irradiated in accordance with a predetermined cardiac phase on the basis of an average cardiac cycle immediately before the imaging to obtain imaging data. The retrospective scan is a method in which electrocardiac waveforms are monitored during the imaging, and only imaging data corresponding to an arbitrary cardiac phase are extracted after completion of the imaging, and used for reconstruction of CT images.
For the electrocardiography-gated scan, there have been reported cases where motions of the heart within the aforementioned time gate width cannot be completely ignored, and therefore motion artifacts remain to induce misdiagnosis. For example, in Non-patent document 2, it was reported that, in the diagnosis of coronary artery stenosis using the retrospective scan, negative predictive value (NPV) exceeded 90%, whereas positive predictive value was only about 70 to 80%. That is, there frequently occurs a case where, in spite of diagnosis of stenosis, stenosis does not actually exist. Therefore, it has a problem that it may invite need for reexamination, or it may lead to an incorrect operation in the worst case.